1. Field of the Invention
The present invention is related to the field of silica crucibles, and more specifically to a silica crucible having a wall in which bubbles and bubble growth are minimized when the crucible is heated to high temperatures.
2. Related Art
Silicon wafers used in semiconductor industries are made from ingots of single crystalline silicon. Such ingots generally are manufactured by the Czochralski (CZ) process. In the CZ process, metallic silicon is charged in a silica glass crucible housed within a suscepter located in a crystal growth chamber. The charge is then heated by a heater surrounding the suscepter to melt the charged silicon. A single silicon crystal is pulled from the silicon melt at or near the melting temperature of silicon.
At operating temperatures, the inner surface of a silica crucible reacts with the silicon melt. The inner layer of a silica glass crucible dissolves into the silicon melt during the CZ process. If there are bubbles in the inner layer, they may open to the melt by dissolution and be a source of particles that may disturb the single crystal structure of the pulled ingot. A bubble-free inner layer has been a critical requirement for a crucible adapted for CZ process use.
The same inventors in the present application are also inventors in pending U.S. application Ser. No. 11/223,158, filed Sep. 8, 2005, which is related to such a crucible and which is incorporated herein by reference for all purposes. The crucible in the pending application has an inner layer that is essentially bubble free and shows minimal bubble growth during the CZ process.
In prior art methods for making such crucibles, including the method in the pending application, when silica grain is fused to form the silica glass crucible, a large volume of gas is released from the grain at the fusion front. The method in the pending application relates to how efficiently this gas is drawn away (through channels that communicate with the inner mold surface) by optimizing the evacuation systems and heating systems. A powerful evacuation system with large diameter pipes and a powerful pump were used to implement an embodiment of the invention in the pending application.
In the prior art, other techniques for minimizing bubbles and bubble growth also use pumps for rapidly drawing gas away from the fusion front. One such technique is illustrated in FIG. 1, which depicts a system, indicated generally at 10, for fusing a silica crucible. The system includes a mold 12 having an inner mold surface 14. Mold surface 14 is formed on a substantially cylindrical vertical wall 16.
A plurality of air channels, like air channels 18, 20 communicate with inner mold surface 14. Each air channel comprises a cylindrical bore that creates a circular opening, like openings 22, 24, on mold surface 14. Each air channel, like channel 20, includes a porous graphite plug, like plug 26, which prevents silica from being drawn from the mold cavity into the air channels. The air channels communicate with manifolds, like manifolds 28, 30, 32, which in turn communicate with a bore 34. A pump (not visible in the drawings) is connected to bore 34. The pump is configured to draw air from the mold cavity via the air channels and ultimately through bore 34 and out of system 10. The pump typically has a capacity of around 350 cubic meters per hour, although this prior art technique may be practiced with pumps outside this range depending on the conductivity of the channels, bores, manifolds, valves, and other structure disposed between mold surface 14 and the pump.
Mold 12 can be rotated by a motor (not shown) about a vertical axis 36. A set of conventional electrodes 38, 40 is vertically movable into and out of the mold interior. The electrodes are connected to a conventional DC power supply 42 that can apply power to the electrodes in a selectable range between about 300 KVA and 1200 KVA. When sufficient power is applied to electrodes 12, 14, an extremely hot plasma gas ball 44 forms around the electrodes.
Mold 12 contains a layer 46 of silica, which is shown partially broken away to expose mold surface 14. Layer 46 includes an inner layer 46a, which fuses when power is first applied to electrodes 12, 14, and an outer layer 46b. Together layers 46a, 46b comprise the wall of a crucible formed in the mold.
Generally describing the operation of system 10, natural quartz grain is placed in mold 12 as it rotates about axis 36. The outer layer of the crucible, i.e., the first grain received in the mold, may be doped with aluminum in a known manner. Once all the grain is received in the mold and shaped with a spatula (not shown in FIG. 1), power is applied to electrodes 38, 40 and the pump (not visible) is turned on. Once the electrodes heat the grain to the point where the grains on the innermost surface of wall 46a begin to fuse, a fusion front forms and proceeds over time from the innermost surface of the crucible to near mold surface 14, where the fusion front saturates. It is desirable to draw gas away from the fusion front at a high rate. This is accomplished in system 10 by creating a high crucible wall 48 that extends substantially all of the way between the upper level of mold openings, like opening 22, to the top of the mold, which is made taller than usual molds to facilitate creation of this tall wall 48.
When fusion first begins, gas is drawn along the radial axis of the mold through the silica in layer 46. But after layer 46a fuses, ambient gas is drawn only through the upper surface of wall 48. The relatively long distance from the upper surface of wall 46 to the first openings, where opening 22 is located, increases the resistance to pump flow. This, in turn, increases the pressure drop between the atmosphere above the upper surface of layer 46, on the one hand, and manifolds 28, 30, 32, on the other hand. As a result, gas formed at the fusion front between layer 46a and the inner surface of mold 14 is rapidly drawn away from the fusion front and into the manifolds by the pump. This minimizes bubbles and bubble growth when the completed crucible is used in a CZ process. Gas may be evacuated at a first pump flow rate until an inner fused layer of about 2.0 mm is formed and then at a second slower pump rate until fusion is complete in accordance with the method described in pending U.S. application Ser. No. 11/223,158, filed Sep. 8, 2005. But the present invention may also be implemented in methods that do not vary the pump flow rate during the entire fusion process.
The upper wall of a crucible so formed may slope in slightly and also may vary in thickness. As a result, after fusion, the upper portion of the crucible is cut off in a known manner. In FIG. 1, after fusion, the crucible is removed from the mold and is cut approximately along plane 49. This removes any defects that may exist in the upper portion. In the process depicted in FIG. 1, the distance from plane 49 to the uppermost portion of the fused crucible is much larger than necessary to remove defects. In other words, the defects typically occur near to the upper rim, and only about 5-20% of the formed crucible needs to be cut off to assure that the remainder of the crucible wall is well formed with a substantially constant thickness. But the process of FIG. 1 includes a much longer portion that is cut off. It is this additional length that increases the resistance to pump flow, which in turn enables use of a smaller flow rate pump to achieve the proper evacuation of fusion gas than if a shorter wall were formed.
There are disadvantages associated with prior art processes. First, forming a crucible in which only the upper 5-20% is cut requires the use of a large pump and large pipes to create and accommodate the pressures and gas flow just described. Using a tall wall in which 30% or more is removed as shown in FIG. 1, permits use of a smaller flow rate pump. But in this case, more silica and dopant is required to make the tall wall on the crucible, which is ultimately cut off. In short, the prior art requires a larger pump or more silica and dopant, and more power to run the larger pump or fuse more silica.